1. Field of the Invention
The disclosure relates to electrical surge protector devices.
2. Description of the Related Art
A semiconductor device can be degraded by excessive voltage. Destructive breakdown or damage can occur in semiconductor devices when subjected to transient over-voltages that persist for only a few microseconds. Transient Voltage Surge Suppression (TVSS) devices, referred to interchangeably herein as surge suppressors and voltage-clamping devices, are commonly known for use in suppressing such over-voltage transients to protect voltage-surge intolerant circuitry. TVSS devices include nonlinear, voltage-dependent resistive elements which display electrical behavior similar to that displayed by a pair of series-connected, back-to-back zener diodes. At normal voltages, below the TVSS clamping voltage level, TVSS devices display a high resistance with a small leakage current. When subjected to large transient voltages (above the TVSS device's clamping voltage), the TVSS device may operate in a low resistance region which increases current flow through the device. When the voltage is increased, the TVSS, due to its characteristics, presents a lower resistance path to current to divert most of the current away from circuitry connected to the TVSS. The potentially destructive surge energy can be dissipated or passed through the voltage-clamping (TVSS) device.
Avalanche diode suppressors, metal oxide varistors (MOVs) and selenium surge suppressors can be utilized as TVSS devices with varying advantages and disadvantages, such as an MOV-based TVSS's inherent susceptibility to failure under certain conditions. MOV devices tend to break down when exposed to an elevated voltage over a long period of time. The breakdown can result in some leakage current through the MOV. Also, MOV components can have a tendency to explode when overheated, often with sufficient explosive power to fracture plastic housings and sheet metal enclosures within which they reside. The explosion can destroy everything within the TVSS housing and may possibly shoot hot black powder through small openings in the housing. Various techniques have been developed to protect MOVs from the causative factors leading to such explosive conditions.
One technique for protecting metal oxide varistors (MOVs) requires adding a current fuse in series with the MOV, which trips to an open state to protect the MOV when particular transient over-voltages are detected. Transients with I2 t ratings that are greater than the fuse rating, but just below the MOV rating will blow the fuse, electrically removing the MOV from the over-voltage condition. Under circumstances where the fuse displays an I2t rating such that commonly occurring transients are insufficient to blow the fuse, i.e., from a few to 10,000 amperes, but of insufficient magnitude to force the MOV to its low impedance state, the MOV may be subjected to overheating, possibly leading to thermal runaway. Steady state, abnormal over-voltage conditions below those at which the fuse will blow may also generate sufficiently high currents through the MOV leading to dangerous overheating.
A second common technique for protecting MOVs from overheating due to abnormal steady state or transient over-voltage conditions utilizes a thermal cutoff device (TCO) coupled in series with the MOV. A TCO is an electrical device that senses the temperature of a surface of an object such as an electrical circuit and trips to a high impedance state (open circuit) at a particular maximum rated temperature. When a TCO is connected in series with an MOV, the TCO senses the surface temperature of the MOV and trips to an open circuit at a particular maximum rated temperature, to cut off voltage to the MOV. Thermal cutoff devices, however, like current fuses are not without problems when used within MOV-protected circuits. In particular, it is extremely difficult, and sometimes impossible to achieve good thermal contact between a surface of the MOV and a thermal cutoff device. Consequently, the MOV may overheat to a point of thermal runaway before the critical temperature is detected and the over-voltage is cutoff from the MOV by the TCO. Further, mismatch problems also may occur between the time constant of the thermal cutoff (i.e., time to open) and the heating/time characteristics of the MOV even when good temperature detection is possible, to render accurate MOV protection unreliable. Both current fuses and thermal cut off devices permanently open upon occurrence of an event and, therefore, must be replaced after such event.
Transient Voltage Suppressors that include MOV-protected circuits may be subjected to over-voltage conditions to determine whether the MOV circuits can fail in a safe manner, that is, without causing damage to equipment or harm to people. In particular, a “Limited current abnormal over-voltage” test involves applying twice-rated normal operating voltage but limits the current to a value below the trip point of any over-current protection device in the power supply line. The test can result in the non-violent heating of the MOVs.
A surge suppression circuit can include three MOV-protected circuits one for each pair of power terminals: Phase and Neutral terminal pair, Phase and Ground terminal pair, and Neutral and Ground terminal pair. The surge circuit may be subjected to a “Limited current abnormal over-voltage” as part of a testing procedure. For example, an over-voltage test is applied across the Phase and Ground terminal pair (as well as the Neutral and Ground terminal pair). This test generates a leakage current that flows through the MOV because of the internal capacitance of the MOV. To prevent the leakage current from flowing to the Ground terminal, a TCO is electrically connected in series with, and physically adjacent, the MOV to detect the leakage current and open the circuit. When an over-voltage test is applied across the Phase and Neutral terminal pair, a leakage current flows from the MOV associated with the Phase and Neutral terminal pair. In this case, the leakage current also flows to the MOV connected across the Neutral to Ground terminal pair.